Mechanical display devices of the type which include a flapper or flag which is moved in order to change the reflective properties of a cell are well known in the art. The method of moving the flapper may comprise magnetic means, electromagnetic means, electrostatic means, or the like. One such system is described in my earlier co-pending application referred to above.
In electrostatic systems of this kind, a fixed electrode which is made of a conductive material covered with an insulative material having one color is sequentially exposed and covered by a Mylar flapper having or giving reflective properties different from the insulative material. Movement of the Mylar flapper is achieved through the use of electrostatic forces.
One of the problems with such prior art systems is the time needed to move a flapper from one position to another. Generally, in order for a system to be desirable, the amount of power necessary to cause the desired movement must be small. If electrical drive is used, the amount of voltage is the limiting factor. In particular, it is desirable to have as low a driving voltage as possible in order to minimize the voltage in the system which may typically include very low voltage devices such as integrated circuits and the like.
Typically, the flapper in an electrostatic display device is made of double metallized Mylar, which is a brand of plastic film, having a thickness typically on the order of 0.0025 cm. with two millionths of a centimeter of aluminum coated onto both sides of the plastic film. As a consequence of the thinness of the film, the pneumatic forces that are exerted by air on the flapper as it moves away from the contact with a fixed electrode are significant. In principal, as the flapper is pulled away from the fixed electrode, a vacuum is created behind the fixed electrode into which air must flow. Because of the viscosity of the air, the vacuum remains for a limited period of time during which the pressure of air on the other side of the flapper bearing against the flapper impedes the movement of the flapper, which is desirably subjected to very low voltages and, accordingly, very low attractive electrostatic forces.